Signed Off

At his Hilarious Pessimist blog, Dalrymple writes of an annoyance received in the post:

…the other day I received a letter informing me that an organization to which I am more or less professionally obliged to belong had received my annual subscription. It contained the following statements, couched in less than polite terms:

This is not a proof that you are a member of the organization to which you have paid the subscription. This is not a proof of identity. This is not a receipt.

What exactly is it, then? I was reminded of Magritte’s picture: Ceci n’est pas une lettre.